In general, the field of the present invention is stem cell cultures. Specifically, the field of the present invention is primate embryonic stem cell cultures.
In general, stem cells are undifferentiated cells which can give rise to a succession of mature functional cells. For example, a hematopoietic stem cell may give rise to any of the different types of terminally differentiated blood cells. Embryonic stem (ES) cells are derived from the embryo and are pluripotent, thus possessing the capability of developing into any organ or tissue type or, at least potentially, into a complete embryo.
One of the seminal achievements of mammalian embryology of the last decade is the routine insertion of specific genes into the mouse genome through the use of mouse ES cells. This alteration has created a bridge between the in vitro manipulations of molecular biology and an understanding of gene function in the intact animal. Mouse ES cells are undifferentiated, pluripotent cells derived in vitro from preimplantation embryos (Evans, et al. Nature 292:154-159, 1981; Martin, Proc. Natl. Acad. Sci. USA 78:7634-7638, 1981) or from fetal germ cells (Matsui, et al., Cell 70:841-847, 1992). Mouse ES cells maintain an undifferentiated state through serial passages when cultured in the presence of fibroblast feeder layers in the presence of Leukemia Inhibitory Factor (LIF) (Williams, et al., Nature 336:684-687, 1988). If LIF is removed, mouse ES cells differentiate.
Mouse ES cells cultured in non-attaching conditions aggregate and differentiate into simple embryoid bodies, with an outer layer of endoderm and an inner core of primitive ectoderm. If these embryoid bodies are then allowed to attach onto a tissue culture surface, disorganized differentiation occurs of various cell types, including nerves, blood cells, muscle, and cartilage (Martin, 1981, supra; Doetschman, et al., J. Embryol. Exp. Morph. 87:27-45, 1985). Mouse ES cells injected into syngeneic mice form teratocarcinomas that exhibit disorganized differentiation, often with representatives of all three embryonic germ layers. Mouse ES cells combined into chimeras with normal preimplantation embryos and returned to the uterus participate in normal development (Richard, et al., Cytogenet. Cell Genet. 65:169-171, 1994).
The ability of mouse ES cells to contribute to functional germ cells in chimeras provides a method for introducing site-specific mutations into mouse lines. With appropriate transfection and selection strategies, homologous recombination can be used to derive ES cell lines with planned alterations of specific genes. These genetically altered cells can be used to form chimeras with normal embryos and chimeric animals are recovered. If the ES cells contribute to the germ line in the chimeric animal, then in the next generation a mouse line for the planned mutation is established.
Because mouse ES cells have the potential to differentiate into any cell type in the body, mouse ES cells allow the in vitro study of the mechanisms controlling the differentiation of specific cells or tissues. Although the study of mouse ES cells provides clues to understanding the differentiation of general mammalian tissues, dramatic differences in primate and mouse development of specific lineages limits the usefulness of mouse ES cells as a model of human development. Mouse and primate embryos differ meaningfully in the timing of expression of the embryonic genome, in the formation of an egg cylinder versus an embryonic disc (Kaufman, The Atlas of Mouse Development, London: Academic Press, 1992), in the proposed derivation of some early lineages (O'Rahilly & Muller, Developmental Stages in Human Embryos, Washington: Carnegie Institution of Washington, 1987), and in the structure and function in the extraembryonic membranes and placenta (Mossman, Vertebrate Fetal Membranes, New Brunswick: Rutgers, 1987). Other tissues differ in growth factor requirements for development (e.g. the hematopoietic system (Lapidot et al., Lab An Sci 43:147-149, 1994)), and in adult structure and function (e.g. the central nervous system). Because humans are primates, and development is remarkably similar among primates, primate ES cells lines will provide a faithful model for understanding the differentiation of primate tissues in general and human tissues in particular.
The placenta provides just one example of how primate ES cells will provide an accurate model of human development that cannot be provided by ES cells from other species. The placenta and extraembryonic membranes differ dramatically between mice and humans. Structurally, the mouse placenta is classified as labyrinthine, whereas the human and the rhesus monkey placenta are classified as villous. Chorionic gonadotropin, expressed by the trophoblast, is an essential molecule involved in maternal recognition of pregnancy in all primates, including humans (Hearn, J Reprod Fertil 76:809-819, 1986; Hearn et al., J Reprod Fert 92:497-509, 1991). Trophoblast secretion of chorionic gonadotropin in primates maintains the corpus luteum of pregnancy and, thus, progesterone secretion. Without progesterone, pregnancy fails. Yet mouse trophoblast produces no chorionic gonadotropin, and mice use entirely different mechanisms for pregnancy maintenance (Hearn et al., "Normal and abnormal embryo-fetal development in mammals," In: Lamming E, ed. Marshall's Physiology of Reproduction. 4th ed. Edinburgh, N.Y.: Churchill Livingstone, 535-676, 1994). An immortal, euploid, primate ES cell line with the developmental potential to form trophoblast in vitro, will allow the study of the ontogeny and function of genes such as chorionic gonadotropin which are critically important in human pregnancy. Indeed, the differentiation of any tissue for which there are significant differences between mice and primates will be more accurately reflected in vitro by primate ES cells than by mouse ES cells.
The major in vitro models for studying trophoblast function include human choriocarcinoma cells, which are malignant cells that may not faithfully reflect normal trophectoderm; short-term primary cultures of human and non-human primate cytotrophoblast, which in present culture conditions quickly form non-dividing syncytial trophoblast; and in vitro culture of preimplantation non-human primate embryos (Hearn, et al., J. Endocrinol. 119:249-255, 1988; Coutifaris, et al., Ann. NY Acad. Sci. 191-201, 1994). An immortal, euploid, non-human primate embryonic stem (ES) cell line with the developmental potential to form trophectoderm offers significant advantages over present in vitro models of human trophectoderm development and function, as trophoblast-specific genes such as chorionic gonadotropin could be stably altered in the ES cells and then studied during differentiation to trophectoderm.
The cell lines currently available that resembles primate ES cells most closely are human embryonic carcinoma (EC) cells, which are pluripotent, immortal cells derived from teratocarcinomas (Andrews, et al., Lab. Invest. 50(2):147-162, 1984; Andrews, et al., in: Robertson E., ed. Teratocarcinomas and Embryonic Stem Cells: A Practical Approach. Oxford: IRL press, pp. 207-246, 1987). EC cells can be induced to differentiate in culture, and the differentiation is characterized by the loss of specific cell surface markers (SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81) and the appearance of new markers (Andrews, et al., 1987, supra). Human EC cells will form teratocarcinomas with derivatives of multiple embryonic lineages in tumors in nude mice. However, the range of differentiation of these human EC cells is limited compared to the range of differentiation obtained with mouse ES cells, and all EC cell lines derived to date are aneuploid (Andrews, et al., 1987, supra). Similar mouse EC cell lines have been derived from teratocarcinomas, and, in general their developmental potential is much more limited than mouse ES cells (Rossant, et al., Cell Differ. 15:155-161, 1984). Teratocarcinomas are tumors derived from germ cells, and although germ cells (like ES cells) are theoretically totipotent (i.e. capable of forming all cell types in the body), the more limited developmental potential and the abnormal karyotypes of EC cells are thought to result from selective pressures in the teratocarcinoma tumor environment (Rossant & Papaioannou, Cell Differ 15:155-161, 1984). ES cells, on the other hand, are thought to retain greater developmental potential because they are derived from normal embryonic cells in vitro, without the selective pressures of the teratocarcinoma environment. Nonetheless, mouse EC cells and mouse ES cells share the same unique combination of cell surface markers (SSEA-1 (+), SSEA-3 (-), SSEA-4 (-), and alkaline phosphatase (+)).
Pluripotent cell lines have also been derived from preimplantation embryos of several domestic and laboratory animals species (Evans, et al., Theriogenology 33(1):125-128, 1990; Evans, et al., Theriogenology 33(1):125-128, 1990; Notarianni, et al., J. Reprod. Fertil. 41(Suppl.):51-56, 1990; Giles, et al., Mol. Reprod. Dev. 36:130-138, 1993; Graves, et al., Mol. Reprod. Dev. 36:424-433, 1993; Sukoyan, et al., Mol. Reprod. Dev. 33:418-431, 1992; Sukoyan, et al., Mol. Reprod. Dev. 36:148-158, 1993; Iannaccone, et al., Dev. Biol. 163:288-292, 1994).
Whether or not these cell lines are true ES cells lines is a subject about which there may be some difference of opinion. True ES cells should: (i) be capable of indefinite proliferation in vitro in an undifferentiated state; (ii) maintain a normal karyotype through prolonged culture; and (iii) maintain the potential to differentiate to derivatives of all three embryonic germ layers (endoderm, mesoderm, and ectoderm) even after prolonged culture. Strong evidence of these required properties have been published only for rodents ES cells including mouse (Evans & Kaufman, Nature 292:154-156, 1981; Martin, Proc Natl Acad Sci USA 78:7634-7638, 1981) hamster (Doetschmanet al. Dev Biol 127:224-227, 1988), and rat (Iannaccone et al. Dev Biol 163:288-292, 1994), and less conclusively for rabbit ES cells (Gileset al. Mol Reprod Dev 36:130-138, 1993; Graves & Moreadith, Mol Reprod Dev 36:424-433, 1993). However, only established ES cell lines from the rat (Iannaccone, et al., 1994, supra) and the mouse (Bradley, et al., Nature 309:255-256, 1984) have been reported to participate in normal development in chimeras. There are no reports of the derivation of any primate ES cell line.